The invention involves an electrode material for electrochemical sensors made of a mixed oxide.
The concentration of unburned combustibles in oxygen-containing gases can be determined by known means in situ in a combustion gas stream by sensors which have two electrodes on a solid electrolyte, e.g. yttrium-stabilized zirconium dioxide. The electrodes react to the measurement gas in different ways. The potential of one electrode is determined to a large extent by the equilibrium oxygen partial pressure of the gas, while the potential of the other electrode is determined predominantly by the partial pressure of the combustible gas, so that a voltage can be measured between the electrodes in the same gas, which voltage is a function of the hydrocarbon concentration. Preferably, gold and alloys of gold and platinum are used as CH.sub.x sensitive electrodes (See, for example, A. Vogel, G. Baier, V. Schuile, Sensors and Actuators 15-16, pages 147-150 (1993)).
It is disadvantageous in arrangements of this type, that gold electrodes are not stable over time in their morphology at the relatively high operating temperatures of the cells (.gtoreq.700 .degree. C.), and as a result the potential adjusting to that is subjected to temporal changes. Another disadvantage is that when the stoichiometric ratio (.lambda.=1) of air to fuel is exceeded, a potential jump is observed which is more or less significant with electrodes of this sort. Moreover, the potential of electrodes of this type is a function of the pretreatment in regard to gas composition and temperature, so that memory effects can be observed, which must be eliminated by subsequent calibration when they are used in sensors.
Furthermore, mixed oxides of the perovskite type are known as electrode materials, which are generally used as oxygen electrodes. Preferably, only oxygen is electrochemically converted at electrodes of this type.
Moreover, perovskite electrode materials are known which are sensitive to the combustible gases. However, these mixed oxides of the perovskite type exhibit the following disadvantages:
By incorporation (e.g., by doping) of allovalent cations into perovskite type compounds of the general formula ABO.sub.3 the oxygen stoichiometry often changes and vacancies in the oxygen sublattice occur, which exhibit considerable mobility. These defects are the cause for an additional, catalytic activity for oxidation processes. Solid electrolyte electrodes, which are constructed from this type of electrode material, convert hydrocarbons from combustible gases in a purely chemical manner using residual oxygen and thereby distort the measurement results.